Process for the homogeneous ester



Patented Sept. 12, 1933 I PROCESS FOR THE HOMOGENEOUS ESTER- IFICATION 0F CELLULOSE HenriLouis Barthelemy, Milan, Italy, assignor to Ruthaldo Company Inc., New York, N. Y.,

1 a corporation of New York No Drawing. Application July 28, 1927, Serial No. 209,161, and in France December 28, 1926 11 Claims. (01.260-101) It is known from scientific literature and from patent specifications that simple or mixed, esters or ethers of cellulose, especially of cellulose acetates and cellulose nitro-acetates, can be obtained by causing anhydrides of organic acids (diluted insuitable solvents, or, not diluted) to actrupon cellulose or on specially prepared nitrocelluloses in the presence of a small amount of a catalyst, which may consist merely of sulfuric acid.

The manner in which the several products are caused, to interact is not without importance on the value of the final result, and it has long been'observed that even by starting with precisely the same'relative quantities of the reacting substances, one is very likely to obtain products which bear no mutual resemblance and even products which very often prove completely useless for technical purposes. the fact that the esterification of cellulose or its derivatives is not a homogeneous reaction, even if acetic acid and acetic, ,anhydride are exclusively used. According to the sequence in which-the individual reagents are added and the manner in which they are employed, one of them may have a greater effect than the other on the cellulose complex. For instance,*in acetylating cellulose in'the manner described in the German Patent Specificationl63,316 or in the French Patent Specification 473,399 the action of the sulfuric aciddiluted merely by the acetic acid, or by acetic acid containinga small amount of acetic anhydride, may easily give rise, at the outset, to' an excessive amount of the hydrolytic phase, to the evident'detriment of the mechanical strength and suppleness of thefinal product, particularly if the steeping temperature rises only a few degrees over-the ascertained optimum, or the contact be too protracted.

s It. may also happen that in the, treatment of cellulose with acetylating mixtures, as described in the German Patent 159,524 or in the French Patents 368,766, 432,046 and 438,649, at the moment of contact with the bath, certain parts of the cotton are more strongly acted upon than others in such a manner that the irregularity of the treatment at starting cannot, be more than imperfectly counteracted by prolonging the duration of the reaction, a circumstance which is again detrimental to'the quality of the final product.

The present invention is based on the discovery that all these difliculties can be obviated and at Y the same time all these causes of failures eliminated; that is to say, that homogeneouscellulose This isdue to lose fibers in order to facilitate the rapidity and uniformity of the penetration of the reagents into the-interior thereof; and

(2). Establishing a succession of slightly differing conditions of chemical equilibrium, governed by the law of mass action, which determines the final state of every phase in equilibrium.

These two principles are applied to the esterification of the cellulose compounds by meansof acetic anhydride in the presence of sulfuric acid as catalyst, or of any other suitable catalyst;

It is known for instance, that cotton consists of hollow, twisted tubes having an outer skin protecting the internal layers, and it is also known that the permeability of this outer skin is considerably lessened by the influence of certain definite treatments, such as prolonged storage in a dry place, or continued exposure to a temperatureof 65 to 70 C. in a hot-air dryer, or boiling for several hours in'toluola phenomenon which is, to, some extent, comparable with that'exhibited to a still greater extent by suchldry vegetables as peas, beans, and lentils, which housewives have to soak prior to cooking.

Such cotton yields products which remain more orless cloudy in an acetone solution, irrespective of the method employed to acetylate them,

the probable reason being the irregular rate of action on the outer skin, or the result of a chemical modification, for instance, an increase in the degree of polymerization of the cellulose complex. On the one 'hand it is necessary to dry the" cotton thoroughly prior to acetylation,

in' order to prevent unduly'lowering the propornon-er anhydride in the baths, and on the other hand it is a frequent occurrence, in industrial practice,th'at 'theifull particulars of preliminary treatment of the cotton dealt'with are not always known. a a f The entirely unexpected and surprising-discovery has been made that not only do cotton fibers completely regain their former capacity of imbibition (in the case of cotton which has prevviously been boiled), but they also become soft and elastic-again, the outer skin moreover being considerably mollified if theyare treated with acetic acid vapor prior to acetylation and are It is known that acetylation of cellulose by means of acetic anhydride represents abimolecular reaction, the velocity of which isproportional to the concentration of the anhydride multiplied by that of the cellulose, the acetic acidi Skin of the cotton fibers take place present exerting no effect upon the velocity.

It can be assumed without committing any great error, that at each and every instant of the process the equilibrium of the whole,.even at the commencement of the reaction, is essentially governed by the law of mass action, even if homogeneity has not yet been attained, seeing that cellulose is a colloidal substance which finishes swelling in the bath after the prior treatment, and therefore the exchanges are effected rapidly by osmosis in view of the very small dimensions of the fibers. It follows that, speaking approximately, and in the case of identical conditions as regards temperature and working procedure, the concentration of the esterified cellulose in a given bath is proportional to the concentration of the acetic anhydride, and inversely proportional to the amount of acetic acid formed.

The present invention consists, in that, in the utilization of said law in order to bring about a succession of different states of equilibrium, which enable the acetylationtoproceed so to speak in successive steps, forming as it were a series of closely approximating steps. The method of operating consists in supplying the bath with only a portion of the anhydride necessary for the acetylation and then. waiting until equilibrium has been nearlyo'r completely established before adding the next portion.

This procedure presents the advantage of effecting the acetylation of each fiber to the same degree, or very nearly, and at thesame time. In fact, if. (especially at the commencement of the operation) one part of the cotton takes up more of the acetylating bath at the moment of immersion than other parts a. circumstance which is chiefly to be ascribed to the difiiculty of stirring the cotton energetically--the differences between the respective degrees of acetylation can never be very great in the present case because the difference in the concentrations of the anhydride between two successive states of the medium is never great, owing to the method of operation employed. v

Moreover, if the operator waits, before adding each fresh portion of the reagents} until the state of equilibrium has been nearly or. completely attained, the result will be that the more highly acetylated portion, if any, is able to undergo retrogression, so that the whole mass is brought, as it were, into the state of uniformity determined by the condition of the bath at the moment under consideration. It is, indeed, well known that the same cotton can be nitrated or denitrated in a bath of the same type by simply varying the amount of water present in the bath; and the same possibility also exists with respect to acetylation since the same principle of saponification, or partial 'retrogression, acetylation is utilized for effecting the solubility in acetone. 1 r

action of the acetic acid vapor and the strong of the Example Into an apparatus heated to to 0., containing 18 kg. of cotton, and communicating with a boiler containing glacial acetic. acid, 20-30 kg. of this acetic acid are passed by vaporization, care being taken'to keep the entire mass thoroughly agitated. The whole is then permitted to cool down to about the temperature of the outer air, which takes about 8 or 12 hours. Due to the liquid acetic acid condensed on the surface of the cotton fibers a swelling and softening of the An acetylation bath is next prepared, containtion 12.5 to 20%, balance being acetic acid) with 0.813 kg.of sulphuric acid (density 1.84). This firstbath is run slowly on to the previously soaked cotton, i. e. cotton previously treated with the vapor of glacial acetic acid and cooled. This cotton is stirred continually the while it is being acetylated. The temperature rises as soon as the cotton commences to collapse but cooling may be effected if necessary.

As socn as a state of approximate equilibrium has been established between the cotton and the bath, 50 kg. of acetic anhydride are added in four portions, each of 12.5 kg, care being taken that sufiicient time elapses between each two of 0 these additions to allow of the complete ornearly complete establishment of'the state of equilibrium determined by the resulting change in concentration. The state of equilibriumcan be determined in any suitable way-for-instance Q10 by examining the fibers by polarized light. If the fibers are uniform and the reaction is properly effected the mass will show a uniform color. under polarized light; but if the action is not complete thefibers will not be uniform, and

stirrers and is composed of a remarkablyho- Q25 .mogeneous cellulose acetate, with an acetyl content-of 47-48%;

The invention is not limited to the foregoing example; 'Catalysers other than sulfuric acid -may be employed; the acetylation may be followed by partial saponification effected by known 'means. Z

'The'expression cellulose used in the claims for purposes of clarity is intended to cover not simply chemically pure cellulose but rather cel- 3 lulose'inthe commercial forms such as cotton, prepared wood pulp, or regenerated cellulose now'or' hereinafter known to be adapted for use as raw material in the manufacture of cellulose esters in the manner indicated in the specifica- 'tion, together with such cellulose compounds or derivatives, such as hydrocellulose, partly esterified cellulose and the like, as are also adapted for suchjpu'rposes.

Having now fully described my invention, what I claim and desire to secure by Letters Patent-is:

1. A process for the purpose specified, consisting in first subjecting cotton to the action of n vaporized acetic acid while thoroughly agitating the mass, allowing the soaked mass to cool, stirring on to the soaked mass a prepared bath containing acetic anhydride with sulfuric acid, and, as soon as a state of equilibrium has been established between the cotton and the bath, adding acetic anhydride in separate portions at such time intervals as will allow complete or nearly complete establishment of a state of equilibrium after each addition.

2. In a process for the purpose specified, subjecting the cellulose to a preliminary treatment with acetic acid vapors so as to soften both the outer skin and the inner layers, then adding the acetylating agent in successive portions.

3. In a process for the purpose specified, subjecting the cellulose to a preliminary treatment with acetic acid vapors so as to soften both the outer skin and the inner layers, then adding the acetylating agent in successive portions, such adclitions being made after a state of equilibrium corresponding to the degree of concentration brought about by the previous addition which has modified the concentration, has been established.

4. A process for obtaining cellulose acetates, consisting in first impregnating cotton with vaporized acetic acid, allowing the treated mass to cool, then subjecting such mass to a mixture containing acetic anhydride and sulfuric acid, and, as soon as a state of equilibrium has been established between the cotton and the bath, finally adding acetic anhydride at intervals until complete or nearly complete equilibrium is produced.

5. A process for the purpose specified, consisting in first subjecting cotton to the action of vaporized acetic acid and cooling the treated mass; subjecting the treated mass to a bath containing acetic anhydride and sulfuric acid, and, when a state of equilibrium has been established between the cotton and the bath, adding more aceticanhydride.

6. A process for the purpose specified consisting in subjecting cotton in an apparatus heated to 55 to 65 C. to vaporized acetic acid whilethoroughly agitating the mass, and allowing the mass to cool to approximately atmospheric temperature, then stirring the mass in a bath containing acetic anhydride with sulfuric acid and thereafter, as soon as a state of equilibrium has been established between the cotton and the bath, adding acetic anhydride in portions at such a time inerval between each two of such additions as will allow of the establishment of an approximate state of equlibrium. e

7. A process for the esterification of cellulose materials; consisting in subjecting cellulose to a preliminary treatment with acetic acid vapors to soften both the outer skin and the inner mass, and then acetylating the treated material by a first treatment with acetic acid, acid anhydride, and a catalyst, and then by successive additions of acetic anhydride thereto, each successive addition being made after an approximate state of equilibrium has been established.

8. A process for the purpose specified; consisting in subjecting cellulose at a temperature of 7 about 55 and 65 C. to the vapors of acetic acid and thoroughly agitating the mass; then allowing the mass to cool to approximately the temperature of the outer air, then stirring on to the mass a bath containing acetic anhydride diluted with acetic acid suflicient to, produce a concentration of from 12.5 to 20% of acetic anhydride, and sulfuric acid, and thereafter, as soon as a state of equilibrium has been established between the cellulosic material and the bath, adding acetic anhydride in portions at intervals, sufficient time elapsing between the successive additions of acetic anhydride to allow establishment of an approximately complete state of equilibrium.

9. A process for the purpose specified; consisting in subjecting 18 parts of cotton to 20 to parts of vaporized acetic acid in an apparatus heated to to C. and thoroughly agitating the mass; then allowing the mass to cool to approximately the temperature of the outer air;

then stirring on to the mass a bathcontaining 70 to 80 parts of acetic anhydride diluted with acetic acid, in order, to give a concentration of from 12.5 to 20% of acetic anhydride, and about 0.8 parts of sulfuric acid with a density of 1.84; and thereafter, as soon as a state of equilibrium has been established between the cottonand the bath, adding 50 parts of acetic anhydride in four portions at intervals, sufficient time elapsing between the successive additions of acetic anhydride to allow of approximate establishment of the state of,

equilibrium.

10. A process for obtaining cellulose acetates, consisting in first impregnating cellulose with vaporized acetic acid, allowing the treated material tocool, then subjecting such material to a mixture containing acetic anhydride and sulfuric acid, and finally adding portions of acetic anhydride at intervals, each successive addition of acetic anhydride'being made after a state of equilibrium between the cellulose and the bath has been established subsequent to the previous addition of acetic anhydride, until approximately complete equilibrium is produced.

11. A process for obtaining homogeneous cellulose acetates; consisting in subjecting cotton in an apparatus heated to 55 to 65 C. to vaporized acetic acid while thoroughly agitating the mass, and allowing the mass to cool; then subjecting the mass to a bath containing acetic anhydride and sulfuric acid and, as soon as a state of equilibrium has been established between the cotton and the bath as shown by the uniform appearance of the fibers under the polarizing microscope, adding more acetic anhydride, the acetic anhydride being successively added in portions until homogeneous cellulose acetates are obtained.

HENRI LOUIS BARTHELEMY. 

